The present invention relates to a method for displaying multiple gray scales on an image display unit which can display all of the gray scales without a saturation area of gray levels having an effective voltage response characteristic and decreasing a driving voltage and sharply decreasing the ratio of a change in the amplitude of the driving voltage between subframes.
Generally, a liquid crystal display, a plasma display panel, or an electroluminescent display is used as the image display unit. A conventional method for displaying the gray scales of these image display units is as follows.
A matrix liquid crystal display device which is widely used as an image display unit at present basically includes scanning electrodes for controlling scanning lines thereof and data electrodes for controlling the display of data on the respective pixels when the respective scanning lines are selected. A voltage averaging method employing a line sequential driving method by means of a multiplexing technique, as shown in FIGS. 1(a)-1(d), is used as the standard method for driving such a simple matrix liquid crystal display device. FIGS. 1(a)-1(c) shows the waveforms of driving signals applied to the scanning electrodes and data electrodes when line sequentially driving the simple matrix crystal display device composed of 2.times.6 pixels by a voltage averaging method and the waveform of the signals applied to the pixels according to the driving signals of the scanning electrodes and data electrodes. In the line sequential driving method, pulses of the voltage Vs (a signal for driving scanning electrodes) are sequentially applied to the scanning electrodes (row numbers 1, 2, 3, 4, 5, and 6) as shown in FIG. 1(a) and pulses of the voltages +Vd and -Vd (a signal for driving the data electrodes) are applied to the data electrodes (column numbers 1 and 2) as shown in FIG. 1(b). Therefore, a device is driven as shown in FIG. 1(d) according to the pixel signals formed by the averaged voltage of the voltages Vs and Vd as shown in FIG. 1(c). Moreover, this method can be used without losing contrast of a picture only in case the response speed of liquid crystal is slow, usually, the response time of a liquid crystal device is about 400 msec.
Therefore, a MLS (multi-line scanning) method or an AA (active addressing) method is being used in the fields requiring a characteristic of high speed response corresponding to the moving speed of a mouse of a computer and to the speed of displaying a moving picture.
FIG. 2 shows the signals applied to the scanning electrodes and data electrodes when driving a liquid crystal display device by applying the MLS method or the AA method. As shown in FIG. 2, the MLS method is the method wherein a plurality of scanning electrodes (F1-F5; Let's assume that the five scanning electrodes are selected out of ten or more.) are simultaneously selected and driven at time t and the AA method is the method wherein all the scanning electrodes (F1-F5; Let's assume that only five scanning electrodes exist and all of them are selected.) are simultaneously selected and driven at time t. At this time, a signal for driving the data electrodes, displayed as G1(t)=-cF1(t)+cF2(t)-cF3(t)+cF4(t)+cF5(t) (c indicates an optional constant) is applied to the data electrode G1, thereby activating two pixels. A plurality of scanning electrodes which are simultaneously driven can be applied to a high speed response liquid crystal display device by increasing a duty ratio of a liquid crystal display device. However, many data voltage levels are required, and a storage unit containing picture data and an operation circuit are additionally required under the present driving circumstances.
For displaying the gray scales by the voltage averaging method adopting the line sequential driving method or the multi-line driving method (or the AA method) there are a frame rate modulation method, an amplitude modulation method, an area division method, a voltage and frame rate modulation method, a voltage amplitude modulation method, and an error diffusion method.
1. Frame Rate Modulation Method for Displaying the Gray Scales.
This method is most widely used for a simple matrix LCD, by which a plurality of subframes are set as a display unit of a screen to be driven. Using this method, gray levels are displayed according to the number of subframes activated among a plurality of subframes. This method is used as a standard for displaying the gray scales because driving expenditure is smallest, since the signals driving the scanning electrodes and those driving the data electrodes all have binary values which can control only the ON and OFF status of the liquid crystal. However, this method has a big problem in realizing the display speed required for displaying a moving picture, i.e., a display frequency of a screen decreases as the number of gray scales displayed increases. Also, a flicker generated due to the lowered screen display frequency deteriorates picture quality.
FIG. 3 shows the frame rate modulation method for displaying the gray scales realizing the eight gray scales using the seven subframes. Here, pulse widths and voltage signals for driving the scanning electrodes are t(s) and Vs, respectively. Also, Vns is a reference voltage and the voltage signals for driving the data electrodes are composed of +Vd and -Vd. As shown in FIG. 3, the method for increasing the frequency of displaying the second through seventh gray levels by increasing the number of subframes is applied, since the frequencies of the screen (the signals driving the data electrodes) are drastically reduced in the second and seventh gray levels. Here, actually, the signals driving the data electrodes are effective only when the signals driving the scanning electrodes are in the "on" state although the frequencies of the signals driving the data electrodes are equal (having a phase difference of 180.degree.) in the second to seventh gray levels shown in FIG. 3. Therefore, the frequencies of the signals driving the data electrodes of displaying the second and seventh gray levels are the lowest.
2. Amplitude Modulation Method for Displaying the Gray Scales.
The amplitude modulation method for displaying the gray scales, as shown in FIG. 4, has an advantage that the signal driving the data electrodes (Y) and the signal driving the scanning electrodes (X), having a pulse width of d, are both composed of only two voltage levels, respectively. However, this method has problems wherein the driving frequencies increase as the pulse width (f) of a data electrode driving signal is divided according to the number of gray scales desired to be realized and wherein the liquid crystal display device cannot respond to the fast signals driving the data electrodes thereby, limiting the number of gray scales which can be displayed.
3. Area Division Method for Displaying the Gray Scales.
The area division method for displaying the gray scales is not used except for special cases because of the problem of lowering resolution, i.e., increase in the number of driving ICs and scanning lines of the screen.
4. Voltage and Frame Rate Modulation Method for Displaying the Gray Scales.
The voltage and frame rate modulation method for displaying the gray scales, as shown in FIG. 5, is a method for allotting subframes to the respective data bits and controlling the amplitudes of the driving voltages considering the weighting values of the respective bits. In the voltage and frame rate modulation method for displaying the sixteen gray scales as shown in FIG. 5, the ratio of the amplitude of the driving voltage Vs to that of the driving voltage Vd is 2.sqroot.2:2:.sqroot.2:1 in the respective frames as the data system is 8:4:2:1. Namely, the difference of the driving voltages between the respective subframes is large and the amplitudes of the driving voltages increase. In this method, in case LCD is driven by the most significant bit data signal under the conditions of the duty 1/240 and Vth 2.0 V the amplitude of the scanning electrode driving signal Vs becomes 35.4 V. In the frame rate modulation method, the amplitude of the scanning electrode driving signal Vs becomes about 22.65 V under the same case and conditions above. It shows an increase of Vs of about 1.56 times as compared with that in the frame rate modulation method. Therefore, since the magnitude difference of the driving voltage levels and that of the subframes becomes larger with an increased number of gray levels, the number of displayed gray levels should be limited. However, this method is estimated to be of possible practical use in the future due to the advantages that it is possible to minimize the number of electrical potentials driving the data electrodes and to sharply reduce the number of subframes in spite of the wide difference in the amplitudes of the driving voltages between the respective subframes.
5. Voltage Amplitude Modulation Method for Displaying the Gray Scales.
The voltage amplitude modulation method for displaying the gray scales is being studied, since it may be used to realize a liquid crystal display device for a high speed response with the development of a method for simultaneously selecting a plurality of electrodes (the active address method). The pulse height modulation (PHM) method, as shown in FIG. 6, is a representative example of this type of application. Here, the pulses of the signals driving the data electrodes (Y), the heights of which are different in the respective half sections (dt/2) of the selected pulse widths (dt) of signals driving the scanning electrodes (X), are applied to the data electrodes. In this method, the expenses for the driving ICs are drastically increased, as countless numbers of electrical potentials for driving the data electrodes are required. Also, there is much to be improved including the limitations of the data processing speed in case the ICs of the analog method are used.
6. Error Diffusion Method for Displaying the Gray Scales.
The error diffusion method for displaying the gray scales is the method for displaying the gray scales by performing space modulation using a picture processing technology. This method is being studied because it allows a sufficient amount of gray scales to be displayed without increasing the expenses for driving the image display unit.
The space modulation method for displaying the gray scales adopting the error diffusion method is performed by an error diffusion system as shown in FIG. 7. In this system, an effective value (u.sub.m,n) obtained by subtracting an error value (e'.sub.m,n), generated at the previous pixels, from the original pixel data (x.sub.m,n) considered as being displayed is approximated into a quantization value (b.sub.m,n) to be used as picture display data. The difference between the effective value (u.sub.m,n) and the quantization value (b.sub.m,n) is set as a new error value (e.sub.m,n) to be diffused into adjacent pixels in a predetermined ratio according to the error diffusion method. These options are sequentially adopted according to the scanning direction thereby displaying desired gray levels. Here, Q(*) denotes a quantizer and h.sub.m,n denotes a low pass filter. The respective values of the error diffusion system are defined by the following formulas: EQU u.sub.m,n =x.sub.m,n -e'.sub.m,n 1.) EQU b.sub.m,n =Q(u.sub.m,n) (quantized) 2.) EQU e.sub.m,n =b.sub.m,n -U.sub.m,n 3.) EQU e'.sub.m,n =h.sub.m,n (e.sub.m,n) (Low-pass filtering) 4.)
The Floyd and Steinberg algorithm is most generally used as a method for diffusing the error values generated in this system to the peripheral pixels, although the Jarvis algorithm, the Judice and Ninke algorithm, and the Stucki algorithm are also widely used therefor. Furthermore, various algorithms other than these are developed and applied according to the application methods.
In the Floyd and Steinberg algorithm, as shown in FIG. 8, the error diffusion is performed for the error to be diffused by 7/16 (eA), 1/16 (eB), 5/16 (eC), and 3/16 (eD), respectively, to the peripheral pixels A, B, C, and D at the pixel P. At this time, the picture data undergoes error diffusion processing in the order shown in the algorithm of FIG. 12. Namely, the picture data of N bits is input, the less significant n bits (n is an integer, i.e., 1, 2, 3, etc.) among the N bits undergo error diffusion processing, and the picture data of (N-n) bits is displayed as a picture.
However, this method has a problem of having a saturation area at the most significant gray level, which is shown in FIG. 9.
FIG. 9 shows substantial gray display states according to the gray level display capability of a display device in the case of displaying 8-bit data using the error diffusion method. Here, "a" depicts a substantial gray level display state in the case of an LCD having two gray levels, in which the gray levels exceeding 128 (a half of the maximum gray level display number of 8-bit data, 2.sup.8 =256) become saturated, thereby unable to discern the gray levels. Lines b, c, and d depict gray display states in the case of LCDs having 4, 8, and 16 gray levels, respectively. Also, "e" denotes the 256 gray scales, which is the limit of displaying the eight bit data.